The most popular liquid crystal cells, particularly for applications such as watch displays, are the so-called field effect liquid crystal devices. These devices employ nematic liquid crystals having positive dielectric anisotropy which align in a direction parallel to the cell's major walls in the off-state and realign in the perpendicular direction in the presence of an electric field. This effect can be viewed through crossed polarizers.
One way to improve the contrast between the on and off states is to add a liquid crystal dye that will change color when an electric field is applied. Several dyes that are soluble in nematic liquid crystals are known; 4"-cyano- or 4"-nitrobenzylidene-4'-(N,N-dialkylamino)-4-aminoazobenzenes have been disclosed in our copending application, Ser. No. 741,538, filed Nov. 12, 1976, U.S. Pat. No. 4,105,654, issued Aug. 8, 1978. These dyes have colors in the orange to red to brown hues.
Cholesteryl p-phenylazophenyl carbonates and carbamates can also be added to positive anisotropy nematic liquid crystals to impart yellow to red colors, as disclosed in our copending application, Ser. No. 642,728, filed Dec. 22, 1975, U.S. Pat. No. 4,032,470, issued June 28, 1977.
2,6-Substituted azonaphthalenes as liquid crystal dyes have been disclosed in copending application of Bloom, Ser. No. 757,374, filed Jan. 6, 1977.
White and Taylor have disclosed various pleochroic dyes that can be added to cholesteric liquid crystal hosts which undergo a phase transition to the nematic state when an electric field is applied, in particular thiazole-substituted phenylazo dyes, see Journal of Applied Physics, Vol. 45, 4718 (1974).
Liquid crystal dyes that can be added to nematic liquid crystals useful in field effect cells having improved contrast and in particular that have colors in the red to blue range, would be highly desirable.